Page 1069 - Advanced Organic Chemistry Part B - Reactions & Synthesis

One aspect of the copper catalytic system that has received attention is the 1045
identity of the active catalytic species. In the case of displacement of aryl bromides by
methoxide ion in the presence of CuBr, it has been suggested that the active species SECTION 11.3
is Cu(I)(OCH , an anionic cuprate. 153 Transition
3 2
Metal–Catalyzed
Aromatic Substitution
I
CuBr + 2 NaOCH 3 [Cu (OCH ) ] – Reactions
3 2
Br
I
III
3
I
[Cu (OCH ) ] – + ArBr [Ar Cu (OCH ) ] – ArOCH 3 + [Cu Br(OCH )] –
3 2
3 2
oxidative reductive
addition elimination
11.3.2. Palladium-Catalyzed Reactions
In Section 8.2.3.2, we discussed arylation of enolates and enolate equivalents using
palladium catalysts. Related palladium-phosphine combinations are very effective
catalysts for aromatic nucleophilic substitution reactions. For example, conversion of
aryl iodides to nitriles can be done under mild conditions with Pd(PPh as a catalyst.
3 4
) ,
Pd(PPh 3 4
(C H ) N CH O CN
CH O I 2 5 3 3
3
) SiCN
(CH 3 3 89%
80 °C
Ref. 154
A great deal of effort has been devoted to finding efficient catalysts for substitution
by oxygen and nitrogen nucleophiles. 155 These studies have led to optimization of the
catalysis with ligands such as triarylphosphines, 156 bis-phosphines such as BINAP, 157
dppf, 158 and phosphines with additional chelating substituents. 159 Among the most
effective catalysts are highly hindered trialkyl phosphines such as tri-t-butyl and
tricyclohexylphosphine. 160 A series of 2-biphenylphosphines 3–6 has also been found
to have excellent activity. 161

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155
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